Capacitors have been well known for many years. In general terms, a capacitor is formed of two isolated conductors of arbitrary shape. Regardless of their geometry, the conductors are referred to herein as plates. The plates of a capacitor can be charged such that a potential difference (voltage) exists between the plates. The ratio between the charge on the plates and the voltage difference between the plates is known as the capacitance of the capacitor.
The capacitance of a capacitor is dependent on several factors, such as the size of the plates, the distance between the plates, and the material between the plates. In order to determine the effect a material will have when placed between the plates, a factor known as the dielectric constant is assigned as a property of the material. For example, the dielectric constant of a vacuum is one, of air is close to one, of gasoline (70° F.) is 2, of industrial alcohol is anywhere from 16-31, and of water (20° C.) is 80.
The effect a dielectric constant has on the capacitance of a capacitor is significant. In fact, thanks to work done by Michael Faraday in the early 1800's, it is known that capacitance is directly proportional to the dielectric constant of the material interposed between the plates.
The fact that the capacitance of a capacitor changes with changes in the dielectric constant of the material between the plates has been exploited in the past to use capacitors as a means for measuring the amount of liquid in a container. The basic idea is to place a pair of opposing plates in a container for storing some sort of liquid or fluid, such as water. Since the dielectric of water is about eighty times that of air, the capacitance of the capacitor will rise as the water rises between the plates. This information can be used by a processor to determine, using a look-up table, interpolation, or a calculation of some kind, the water level in the container.
The principle drawback to capacitance gauges such as the one described above is that significant inaccuracies can occur due to changes in the dielectric constant in the liquid in the container. For example, it is well-known that the dielectric constant of many materials can vary with changes in temperature and as contaminants are introduced into the material. Another problem can occur if the container is used for a variety of types of liquids. In the example above, the container could be filled with gasoline having a dielectric constant of two at some point instead of water. In that case, when the processor makes a calculation to determine the level of gasoline in the container based on the dielectric constant of water, which is 80, the result would be completely unreliable.
This is similar to a situation that happens with some types of vehicles, where the type of fuel put into the fuel tank can vary, which causes variations in the dielectric constant of the fluid in the fuel tank. For example, as noted above the dielectric of alcohol is as much as fifteen times that of gasoline, making the difference between gasoline and gasohol significant in terms of dielectric constants. In addition, sometimes additives such as deicers are introduced into the fuel tanks, and inevitably contaminants accumulate in the fuel tanks as well. All of this contributes to variations in the dielectric constant of fluid in the fuel tank that can cause inaccurate indications of fuel levels if not accounted for.